Light gauge, hot dip metal coated steel products



March 21, 1967 L. M. BERNICK ETAL 3,

LIGHT GAUGE, HOT DIP METAL COATED STEEL PRODUCTS Filed April 22, 1963 2 sheets-55am 1 co LD REDUCTION CLEAN l2- HEAT TREATMENT "l l I TEMPER ROLL HOT DIP AFPLICATION 14.

OF METAL COATlNG SECOND cam a Revue-nor! CLEAN CHEMICAL -"'Z0 TREATMENT Inven'fiors Leslie M. Ber-nick William C.Sievert s T C U D O R L E m LS D mm KC um mm M m m LH E w A G T H G I L March 21, 1967 2 t e e h s t e e h s 2 Filed April 22, 1963 .wmSQ- P om whmluQix COLD REDUCTION 370cc REDUCTIDH Inveniors Leslie Bernick William CuSeivePi: fiflmww United States Patent Ofiiice 3,309,906 Patented Mar. 21, 1967 3,309,906 LIGHT GAUGE, HOT DIP METAL COATED STEEL PRODUCTS Leslie M. Bernick, Calumet City, 111., and William C.

Sievert, Chesterton, Ind, assignors to Inland Steel Company, Chicago, 111., a corporation of Delaware Filed Apr. 22, 1963, Ser. No. 274,425 3 Claims. (CI. 72-47) The present invention relates generally to the production of very light gauge steel strips having a hot dip protective metal coating on at least both sides thereof, and more particularly to the production of light gauge steel strips having thereon a coating of a hot dip protective metal, such as zinc, aluminum, terne or the like protective metal coating.

In order to enable the steel industry to compete on equal terms with suppliers of other materials now available, there has been a need for thin, light weight, stiff steel sheet products having a protective metal coating thereon which have desirable fabrication properties and which can be sold at a reasonable price. Steel sheet products of the above type for which there is a present need are light gauge hot dip galvanized and aluminized steel sheets having a final thickness of about 0.0140 inch or below and preferably ranging in thickness between about 0.0060 and 0.0138 inch.

At the present time the lightest gauge galvanized steel strip sheets generally produced on any continuous hot dip galvanized line is about 30 gauge. However, since thin gauge hot dip galvanized steel sheet having a final thickness in the range of 0.01 inch, if produced in accordance with the present method of producing galvanized sheet materials, would be unsuited for many uses of galvanized sheet material and could not be produced on presently available continuous galvanizing apparatus, an improved method of making thin gauge hot dip coated steel sheet material of the above type is needed.

It is therefore an object of the present invention to provide an improved method of economically producing thin gauge steel sheet material having a hot dip protective metal coating on at least one side thereof.

It is also an object of the present invention to provide an improved method of economically producing hot dip metal coated thin gauge steel sheet material on presently available in-line continuous production apparatus.

It is a still further object of the present invention to provide an improved method of economically producing thin gauge hot dip galvanized and aluminized steel sheet material.

Other objects of the present invention will be apparent to those skilled in the art from the following detailed description and claims when read in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a block diagram flow sheet illustrating one embodiment of the present invention.

FIGURE 2 is a graph showing the relationship between the hardness values of a steel strip and the percentage of cold reduction of said strip effected in a final cold rolling operation.

FIGURE 3 is a graph showing the relationship between the impact tensile percent elongation and the percentage of cold reduction of said strip effected in a final cold rolling operation.

As a result of extensive work regarding the production of thin gauge hot dip metal coated steel sheets, it has been discovered that while certain of the physical properties of a cold reduced hot dip coated steel sheet are directly related to the amount of cold reduction required to obtain the desired finished gauge or thickness, other physical properties of these sheets change more rapidly than could be anticipated for a given amount of cold reduction in thickness of a steel strip. From the hardness curve of FIG. 2 of the drawing, for example, it will be evident that above about 60% cold reduction of the thick ness of the initial steel strip the hardness of a cold reduced steel strip increases sharply with the increase in percentage of cold reduction. Similarly, from FIG. 3 of the drawing it will be apparent that the relationship between percent elongation and percent cold reduction is not linear, and that within the range of from about 20% up to about 60% cold reduction the slope of the device appears to be fairly constant and the percent of elongation decreases with an increase in percent of cold reduction. The foregoing characteristics of cold reduced steel strips assume importance in connection with the fabrication of articles therefrom. It has been found from the 180 bend test and the Pittsburgh lock seam test conducted on thin gauge galvanized steel strips that when a zinc coated steel strip, for example, is cold reduced in thickness above about 60%, the base metal broke when the strip was bent in the longitudinal direction, and below about 60% the strips did not break when bent in either the transverse or longitudinal direction. Thus, it has been found that by final cold reducing galvanized steel strips a maximum of about 60%, and preferably 50%, in producing thin gauge galvanized steel sheet material, there is achieved good adherence of the zinc coating as evidenced by bending of the reduced strips 180 in both a longitudinal and transverse direction.

In general, it has been found that in producing very thin gauge steel sheet material having a hot di protective meal coating thereon, such as zinc or aluminum, that the percentage of cold reduction of a steel strip should be within the range of about 20% to about 60% and preferably effecting between 25% and 50% cold reduction. Within the above general range of percentage of cold reduction, the hardness values of the finished coated strip are satisfactory and of the same general magnitude for the range of reductions specified, the product has ductility, and the product can take a to bend in the transverse direction without breakage.

In practicing the present invention, it has been found preferable to cold reduce on conventional apparatus uncoated steel strips to a thickness which is about 20% to 60%, and preferably from 25 to 50%, in excess of the finally desired finished thickness, anneal the cold reduced strips, and apply a hot dip protective metal coating, such as zinc or aluminum, to provide a hot dip protective metal coating having a thickness between about 20% to 60% in excess of the desired coating thickness. Thereafter the hot dip coated strip is subjected to a second cold reduction treatment by any suitable cold rolling apparatus to cold reduce the thickness of the coated strips between about 2.0% to 60%, and preferably between 25 and 50% to a finished thickness below about 0.014 inch. In each instance the thickness of the finished strip is between about 80% and 40% of the thickness of the coated strip before coating the strip with a protective metal, and it will be evident that by proper selection of the thickness of the strip to be coated, a wide range in the thickness of the finished plate can be produced.

A practical limiting factor when considering the gauge of steel strip to be used in the present invention, and particularly the thickness of the annealed strip which is to be coated prior to a second or final cold reduction treatment, it is the minimum gauge of steel strip that can be processed through the continuous metal coating apparatus. In a continuous galvanizing line, for example, the minimum gauge steel strip which can be processed on the presently used apparatus is about 30 gauge. Thus, the possible range of gauges or thickness which can be produced in accordance with the present invention, for example, when a 35% cold reduction in the second or final cold reduction treatment is achieved, would be a final gauge thickness ranging between about .0122 inch and .0138 inch. As a specific example where a 35% cold reduction of a galvanized steel strip is effected, an annealed steel strip having an initial thickness of .0133 (corrected for 1.5 ounces per square foot coating weight) and having an initial gauge after galvanizing of .0157 inch will when subjected to a 35% cold reduction have a finished gauge thickness of .0102 inch. With a 50% cold reduction, the final gauge thickness would range between about .008 to .0138 inch. As a specific example where a 50% cold reduction of a galvanized strip is effected, an annealed steel strip having an initial thickness (before final cold reduction) of .0125 inch (corrected for 2.0 ounces per square foot coating weight) and having an initial gauge after galvanizing of .0157, will when subjected to a 50% cold reduction have a finished gauge of substantially 0.008 inch.

While no arbitrary lower limit can be set for the finished thickness of the thin gauge hot dip. coated steel strip, it is generally preferred in making thin gauge hot dip zinc coated steel strips to have the thickness of the steel strip range from between about .008 to .0137 inch. The optimum finished thickness of the coated steel strip will in each instance be determined by the particular use requirements and a cold reduced annealed steel strip can be selected for coating which on being reduced, preferably between about 25-50%, will have the desired final thickness.

It has been found that the cold reduction of a hot dip metal coated steel strip up to about a 35% reduction can be most conveniently carried out on a single stand or twin stand, four-high temper mill or Steckel mill, using rolling lubricant, such as palm oil or the like. Any percentage of cold reduction above about 35%, however, must generally be carried out on either some type of tandem mill, such as a two-stand or five-stand tandem mill, or on a reversing mill. It will be appreciated, of course, that a 20% to 35% cold reduction can also be carried out on the latter apparatus, if desired.

A double cold reduced light gauge hot dip galvanized sheet product produced in accordance with the foregoing preferred procedure wherein cold reduction of between 25% and 50% is effected in the second or final cold reduction step has a grey appearance without spangles, has a hardness in the full hard range of Rockwell 75 (30 T-scale), has an ultimate strength range from about 85,000 to 95,000 pounds per square inch in the longitudinal direction, and has the required degree of ductility, as indicated by impact tensile strength values. The light gauge galvanized coating and the supporting steel base can take up to an 180 bend in the transverse direction without breaking and has phosphating characteristics and corrosion resistance similar to that of normal galvanized sheet with the same coating weight.

In order to further illustnate the present invention, the following specific examples are given, without, however, limiting the invention to the use of the specific materials or conditions described.

Example I A standard steel hot mill band having a thickness of about 0.080 inch is cold reduced on a five-stand tandem mill to form a steel strip having a thickness of about 0.0236 inch thick (24 gauge). The full hard 0.0236 inch strip thus formed having a Rockwell hardness (30 T-scale) of about 80 is cleaned by passing through a continuous cleaning line 11, followed by convention box annealing heat treatment 12 to restore the ductility lost when the strip was cold reduced. The annealed strip is then temper rolled 13 to provide a suitable surface for galvanizing. Thereafter, the strip is passed through a conventional continuous hot dip galvanizing line 14 wherein a coating weight of about 1.50 ounces zinc per square foot is applied. The galvanized strip is then cold reduced on a cold reduction tandem mill 16 using palm oil as the rolling lubricant to effect about a 50% reduction in the thickness of the strip, preferably in increments of about 25% reduction per pass, and provide a steel strip reduced to a finished thickness of 0.0110 inch and having the zinc coating simultaneously reduced to about 0.75 ounce of zinc per square foot. The light gauge galvanized plate thus produced has a Rockwell hardness of (30-T-sca1e) on Olsen ductility of .160 inch, and an ultimate longitudinal tensile strength of 89,390 pounds per square inch.

Example II A standard steel hot mill band having a thickness of about 0.080 inch is cold reduced on a five-stand tandem mill 10 to form a steel strip having a thicknsss of about 0.0173 inch thick (28 gauge). The full hard 0.0173 inch strip thus formed having a Rockwell hardness (30 T-scale) of about is cleaned by passing through an alkaline detergent solution at a cleaning station 11 and heat treated by passing through a continuous heat treating zone 12 having a protective atmosphere against oxidation and wherein the strip is heated at a temperature ranging between 1000 F. to 1800 F. to restore the ductility lost when the strip was cold reduced. Thereafter, the strip passed through a hot dip galzanizing bath 14 wherein a coating weight of about 1.50 ounces zinc per square foot is applied on a conventional continuous hot dip galvanizing line. The galvanized strip is next cold reduced on a twin stand, four-high temper mill 16 using palm oil as the rolling lubricant to effect about a 35% reduction in the thickness of the strip, preferably in increments of about 17% reduction per pass and provide a steel strip reduced to a finished thickness of 0.0113 inch and the coating weight is simultaneously reduced to about 0.97 ounce zinc per square foot. The light gauge galvanized plate thus produced has a Rockwell hardness of 75 (30 T-scale), an Olsen ductility of .171 inch, and an ultimate longitudinal tension strength of 87,399 pounds per square inch.

Example III A standard steel hot mill band having a thickness of about 0.080 inch is cold reduced on a five-stand tandem mill 10 to form a steel strip having a thickness of about 0.0236 inch thick (24 gauge). The full hard 0.0236 inch strip thus formed having a Rockwell'hardness (30 T- scale) of about 80 is cleaned by passing through an oxidizing furnace and then heat treated by passing through a continuous heat treating zone 12 having a protective atmosphere against oxidation and wherein the strip is maintained at a temperature between about 1200 F. and 1800 F. to restore the ductility lost when the strip was cold reduced. Thereafter, the strip is passed through a hot dip aluminizing bath 14 wherein a coating weight of about 1.00 ounce of aluminum per square foot is applied on a conventional continuous hot dip aluminizing line. The thin gauge plate is next cold reduced on a twin stand cold reduction tandem mill 16 using palm oil as the rolling lubricant to effect about a 45% eduction in the thickness of the strip, preferably in increments of about 22% reduction per pass. The steel strip is reduced to a thickness of 0.0132 inch and the coating weight is simultaneously reduced to about 0.50 ounce aluminum per square foot. The light gauge aluminized plate thus produced has a Rockwell hardness of 74 (30 T-scale), an Olsen ductility of .168 inch, and an ultimate longitudinal tensile strength of 87,496.

In the present invention it will be understood by those skilled in the art that the hot mill band comprising the base strip is a standard relatively low carbon steel having a carbon content between about .002% C. and .10% C. When the steel base has a carbon content at the lower portion of the foregoing carbon range, such as a carbon content of .01% C. or below, the percentage reduction in the final cold reduction step can be at the upper end of the specified percent reduction range (e.g. 5060% reduction in thickness) and the thin gauge product obtained will have the desired stiffness and formability. If a steel base having a higher carbon content is used, a thin gauge product having the same desired properties is obtained by cold reducing the hot dip coated strip in the final cold reduction step to a lesser degree (e.g. 2540% reduction in thickness). It should also be understood that by employing a slower but more effective box annealing heat treatment, it is possible to reduce the thickness of any of the above relative low carbon steel bases within the range of about 20 to 60 percent reduction and obtain a thin gauge hot dip coated product having the desired characteristics.

It will also be appreciated by those skilled in the art that the very thin steel sheet having a hot dip protective metal coating thereon when produced by the process of the present invention has the necessary strength and stiffness required for use in the fabrication of containers, awnings, ducts and the like articles for which conventional thicker gauge galvanized and aluminized steel plate is presently used.

We claim:

1. A process of producing a thin gauge steel strip having a hot dip protective metal coating thereon which comprices; reducing a hot mill steel band in a first cold rolling operation to form an intermediate steel strip having a thickness substantially greater than the final desired thickness of said thin gauge steel strip, subjecting said steel strip to an annealing treatment to restore the ductility lost in said first cold rolling operation, treating said steel strip to provide a hot dip protective metal coating thereon, passing said metal strip with the metal coating through a final cold reduction operation in the presence of a rolling lubricant to effect a percentage reduction in the thickness of the said strip between about 25 and 50 percent, and thereafter subjecting said steel strip to chemical treatment to remove objectionable surface contamitions; whereby a strong thin gauge hot dip metal coated steel plate is produced.

2. A process of producing a very thin gauge hot dip galvanized steel strip which comprises; reducing a hot mill steel band in a first cold rolling operation to form an intermediate steel strip having a thickness substantially greater than the final desired thickness of said thin gauge steel strip, subjecting said steel strip to an annealing treatment to restore the ductility lost in said first cold rolling operation, hot dip galvanizing said strip to provide a protective zinc coating thereon by subjecting said strip to a conventional hot dip galvanizing treatment, passing said strip with said zinc coating through a final cold reduction operation in the presence of a rolling lubricant to effect a percentage reduction in the thickness of the said strip between about 25 and 50 percent to a thickness between about .008 and .0138 inch, and thereafter subjecting said galvanized steel strip to chemical treatment to remove objectionable surface contamination; whereby a strong thin gauge hot dip galvanized steel strip is produced.

3. A process of producing a very thin gauge hot dip aluminized steel strip which comprises; reducing a hot mill steel band in a first cold rolling operation to form an intermediate steel strip having a thickness substantially greater than the final desired thickness of said thin gauge steel strip, subjecting said steel strip to an annealing treatment to restore the ductility lost in said first cold rolling operation, subjecting said steel strip to a conventional hot dip aluminizing treatment to provide a protective aluminum coating thereon, passing said strip with said aluminum coating through a final cold reduction operation in the presence of a rolling lubricant to effect a percentage reduction in the thickness of the said strip between about 25 and 50 percent, and thereafter subjectin said aluminized steel strip to chemical treatment to remove objectionable surface contamination; whereby a strong thin gauge aluminized steel strip is produced.

References Cited by the Examiner UNITED STATES PATENTS 2,797,476 7/1957 Sendzimir 29528 3,057,050 10/1962 Hodge et al. 29-1962 3,118,223 1/1964 Schull et al. 29528 3,188,734 6/1965 Davis 29528 CHARLES W. LANHAM, Primary Examiner.

L. A. LARSON, Assistant Examiner. 

1. A PROCESS OF PRODUCING A THIN GAUGE STEEL STRIP HAVING A HOT DIP PROTECTIVE METAL COATING THEREON WHICH COMPRICES; REDUCING A HOT MILL STEEL BAND IN A FIRST COLD ROLLING OPERATION TO FORM AN INTERMEDIATE STEEL STRIP HAVING A THICKNESS SUBSTANTIALLY GREATER THAN THE FINAL DESIRED THICKNESS OF SAID THIN GAUGE STEEL STRIP, SUBJECTING SAID STEEL STRIP TO AN ANNEALING TREATMENT TO RESTORE THE DUCTILITY LOST IN SAID FIRST COLD ROLLING OPERATION, TREATING SAID STEEL STRIP TO PROVIDE A HOT DIP PROTECTIVE METAL COATING THEREON, PASSING SAID METAL STRIP WITH THE METAL COATING THROUGH A FINAL COLD REDUCTION OPERATION IN THE PRESENCE OF A ROLLING LUBRICANT TO EFFECT A PERCENTAGE REDUCTION IN THE THICKNESS OF THE SAID STRIP BETWEEN ABOUT 25 AND 50 PERCENT, AND THEREAFTER SUBJECTING SAID STEEL STRIP TO CHEMICAL TREATMENT TO REMOVE OBJECTIONABLE SURFACE CONTAMITIONS; WHEREBY A STRONG THIN GAUGE HOT DIP METAL COATED STEEL PLATE IS PRODUCED. 